This invention relates to a process for adjusting the level of water or water soluble additives in aqueous-based powder injection molding compounds for the purpose of recycling scrap material, controlling shrinkage or rehydrating dry feedstock. Depending on the objective, the process may require material granulation equipment, equipment for the addition or removal of water and mixing equipment.
Currently, water-based metal and ceramic injection molding compounds containing an agar binder are supplied to customers with a very tight tolerance on solids level of xc2x10.20 wt %. Meeting this specification requires shipping in waterproof containers and maintaining temperatures in a narrow range during shipping and storage. If these compounds were manufactured in pellet form and could be dried after compounding and shipped dry, shipping and storage could be greatly simplified. The pellets would then need to be rehydrated to specification levels prior to use in an injection molding process. Rehydrating the pellets at a user""s facility would require additional equipment. However, a sophisticated user would be able to use water level or water-soluble additives to make minor adjustments to improve processability or to control the dimensions of the final heat-treated or sintered part.
The same process can be used to recycle scrap parts in the form of runners, rejected parts, start-up scrap and purge scrap generated from an injection molding process prior to sintering, known as xe2x80x9cgreenxe2x80x9d material. There is a strong economic incentive to recycle this green material, as it is fairly expensive. The green scrap material is first granulated and then rehydrated to specification levels, as discussed above with respect to the dry pellets. The recycle process results in an increase in flow as evidenced by the higher spiral flow results and a decrease in green strength. This loss of green strength, which may compromise removal from the mold, can be reversed by the addition of a water soluble additive such as calcium borate.
Shrinkage levels for powder injection molding materials is very high, typically about 16%. A common problem for injection molding manufacturers is meeting close dimensional tolerances for sintered parts. Some of the reasons for variation in shrinkage include (i) variation in material composition, (ii) variation in molding or sintering conditions, and (iii) improper design of the mold. Manufacturers generally do not want to make expensive changes to their molds, especially if they suspect that the variations may be transient. Also, they may not be willing or may not know how to make the required adjustments in molding and sintering conditions to achieve dimensional control. A better and more cost effective way to control shrinkage is to adjust the water level of the compound.
The present invention solves the problem of water loss in recycled parts or dry compound pellets by rehydration of the granulated recycled parts or pellets to the appropriate water level required to meet specification, in order to assure the proper material solids level. The invention also solves the problem of shrinkage control by allowing the water level to be adjusted either up or down to yield the desired shrinkage.
The present invention provides a process for adjusting the water content of a water-based injection molding compound to specified levels in order to produce desired processing characteristics and product properties including the steps of providing a metal or ceramic powder-based molding compound, providing apparatus for the addition or subtraction of water to the compound, and measuring the water content in the compound to produce an optimum balance of injection molding process characteristics and product properties.
The present invention also provides a process for adjusting the level of water-soluble additives in a water-based injection molding compound to specified levels in order to produce desired processing characteristics and product properties including the steps of providing a metal or ceramic powder-based molding compound, providing apparatus for the addition or subtraction of water-soluble additives to the compound, and measuring the content of water-soluble additives in the compound to produce an optimum balance of injection molding process characteristics and product properties.
The molding compounds may be comprised of either recycled scrap material before being sintered or dry, virgin feedstock material.